Got a question that hopefully can provide a fairly general answer to.

I was talking with an individual and the topic came up about exhaust pipe size vs power output of a said engine and is prompting this question.

I personally feel that for my F150 a stock spec exhaust which is 2 1/2" Y pipe into a round 6" muffler and out a 2 1/4" tail pipe should be adequate for a street engine that wont turn over 5,500 rpm. But the question I have is, would that be the only aspect I should look at or should I look at changing the size?

What I currently have is a set of Hedman street headers for the 86 - 96 F150/Bronco with a 302, Part # 89470 which on hedman`s website state is a 1 1/2" primary tube diameter with a OE ball and socket flange collector that is listed as a 3" collector attachment. The OE Y pipe, which I am trying to source one from Hedman without the precat is 2 1/2" like OE and the muffler I have is the MagnaFlow XL 3 chamber muffler #13645. This is a 409 stainless steel 6" round muffler that is 2 1/4" inlet/outlet that has a 27" case exterior length and 33" over all length. I picked this as this is roughly the same dimensions as the OE Walker replacement muffler for my truck dimensions wise, just doesnt have a 2 1/2" inlet and a 2 1/4" outlet so I opted for the 2 1/4" inlet/outlet so I can place a reducer in the pipe infront of the muffler. Tail pipe I have is a 83-86 351W equipped F150 that is 2 1/4" from Walker as the 81-82 302 F150 tail pipe they only offer in the incorrect 2" size.

With that all said with 1 1/2" primary and 3" collector attachment with OE ball and socket flange shorty headers, with a OE 2 1/2" Y pipe with 2 1/2" straight pipe ending in a 2 1/2" to 2 1/4" reducer to a 6" round muffler that has a 27" long case to a 2 1/4" tail pipe, what kind of power could this exhaust system support for a engine that wont turn over 5,500 rpm? Truck has a C6 and I will have the transmission shop that will build my transmission adjust the governor to shift at no more than 5,500 rpm. Redline should be around 6,000 but it would never be seen.

Im not real big on exhaust I am only doing this as this is a daily driver but in talking with this other individual I started wondering if my OE exhaust would even be capable of supporting a 302 based engine build making 300hp which is my goal, which I feel it should as the exhaust was also used in the same size on the 351W equipped trucks which is pushing more exhaust volume than a 302/306 displacement would. Realistically my engine might make closer to 400 based off other builds that are similar for the engine but I am only seeking 300hp for my build. So I am open to hearing what people think, I wont run dual exhaust as it is too hard to do on these trucks without having one pipe considerably longer than the other going from driverside to passengerside then back to driver side. I would be open to going with a larger exhaust system but I would still have to retain my OE ball and socket flange which uses 2 1/2" tubing and I dont believe going from 2 1/2" tubing to say 3" tubing would make a difference as the bottle neck would still be the collector. Then there is the headers, I dont feel the street headers I got would even be capable of supporting larger exhaust system without drastically slowing the exhaust flow down. For my engine in the off chance it makes a difference, it wont be a stroker, I ruled stroker out and this will just be a bored 302 producing a 306. I cant justify the cost of a stroker rotating assembly for a daily driver and then having to toss the 50oz imbalance balancer I already bought in favor of a 28oz imbalance balancer.

 

This may help

 

Ive seen that chart when I was looking, based off that chart I wouldnt be able to make over 200hp with the 2 1/2" into 2 1/4" which I find hard to believe. Some of the other charts Ive seen showed 275hp for single exhaust but none of them are taking displacement into account, a engine is a air pump basically and based off displacement can move only so much air for a given rpm. I found one chart years ago that broke it down by displacement but cant find it again, I want to say for a 302 to 350 cid a 2 1/2" single pipe was I think listed as 325 hp capable.

But thanks for the chart anyways, this is just something that got me thinking and I am now wondering if my 306 build with a goal of 300hp that may make 350hp to 385hp I am wondering if my exhaust would hinder that power figures for a small displacement V8 that wont turn over 5,500 rpm in normal driving.

 

It seems to me you will lose velocity, with each side of engine only supporting 150 of the 300hp, and using 2 1/2, instead of 2 1/4, to keep velocity up in the lower RPM's, how much more could you lose, then if you think torque, how much will you lose, with just a little on the big side pipe(in upper RPM's), and we need torque(low RPM's) to get our trucks moving, not a screamer and 5,000 rpm launches.
I think you seen this

 

Ive seen that chart but I honestly do not believe one needs 3" pipes single for 250hp. I know many 350`s rated at 250hp with OE 2 1/2" single exhaust. Unless I am looking at it wrong, I came across that chart how ever in my search and not sure if I want to really believe that. From my understanding you want to keep exhaust pipes as small as possible to keep velocity up as well as to keep the exhaust hot. Mine if I do it how I had planned which is the way OE did it, its 2 1/2" from Y pipe to the muffler, inlet side of the muffler is 2 1/2" then outlet of the muffler is 2 1/4" and the tail pipe is 2 1/4". I could get a 2 1/2" muffler variant of the 2 1/4" one I have and just do it all in 2 1/2" and just have someone use my OE 2 1/4" tail pipe as a pattern to bend me a 2 1/2" one. But still even using this chart it appears that the smallest size I could go with is 3" for single exhaust.

Wish I could find that one I found years ago you punch in max RPM and displacement and it would tell you the size exhaust pipes based off displacement and engine speed.

 

Well I came across Bassani Y pipes for 86 - 95 F150s that are offroad which is nice but doesnt state if they have OE flange collectors or not but they are listed as 2 1/4" pipes from the manifolds/headers to the Y then 3" pipe after the Y. I also found magnaflow makes a 7" round XL 3 Chamber muffler just like what I had already bought in 6" round but this one is slightly longer but has a 3" in/out so only thing I would have to do is find a local exhaust shop that can custom bend me a tail pipe in 3" using my 2 1/2" for a pattern then they can cut off the hanger and attach it to the 3" pipe.

Only thing is by charts 3" single would still only be good for 300hp and if my motor makes more than 300hp then I am still screwed unless I run a dual setup which would require more money and more material then the driverside will be considerably longer having to shift the pipe from driverside to passengerside then back to driverside.

 

While undoubtedly not optimum I think that these charts can be exceeded by a pretty wide margin if the pipe has very few bends and none that are crimped. For instance; Panteras usually have 2.5 inch exhaust pipes. I've made 618 horsepower from a 383 Cleveland through two 2.5 inch pipes with mufflers. My truck has a single 3 inch out and two 2.5 inch pipes back to the muffler. That engine made 330 horsepower on the dyno. As long as the muffler isn't too restrictive it seems like that system would be adequate according to the chart.

 

Don't limit yourself.